1. Statement of the Technical Field
The present invention relates to the field of wireless communications and more particularly to a method and system for the fractional reuse of frequencies in a manner that adapts to channel environment of the wireless communication network.
2. Description of the Related Art
Wireless communication networks, such as cellular networks, operate by sharing resources among the mobile terminals operating in the communication network. As part of the sharing process, resources relating to which channels, codes, etc., are allocated by one or more controlling devices within the system. Certain types of wireless communication networks, e.g., orthogonal frequency division multiplexed (“OFDM”) networks, are used to support cell-based high speed services such as those under the IEEE 802.16 standards. The IEEE 802.16 standards are often referred to as WiMAX or less commonly as WirelessMAN or the Air Interface Standard.
OFDM technology uses a channelized approach and divides a wireless communication channel into many sub-channels which can be used by multiple mobile terminals at the same time. These sub-channels and hence the mobile terminals can be subject to interference from adjacent cells because neighboring base stations can use the same frequency blocks.
As such, a method is needed to adjust the output power of the base stations to a value which enables mobile terminals at the cell edge to still communicate at an acceptable rate while not creating too much interference in neighboring cells. A method, described below, has been proposed to reduce interference caused by neighboring base stations using the same frequency blocks and thereby help to increase mobile terminal throughput and overall network capacity.
For distant mobile terminals, i.e., mobile terminals at the edge of a cell, a base station has to use more transmission power in order to reach them. Mobile terminals close to the base station require much less transmission power to receive the signal. Because known mobile terminals only transmit and receive on some but not all sub-channels of the frequency band, transmission power of sub-channels used by mobile terminals close to a base station can be lower than the transmission power of sub-channels used by mobile terminals at the cell edge. In practice, the reduced transmission power for sub-channels used by mobile terminals close to a base station creates less interference for mobile terminals close to other base stations.
It is known that a combination of high and low power sub-channels can be used to increase the overall coverage of the network compared to networks which use the same transmission power for all sub-channels. Base stations can be organized in a way to use the same set of sub-channels to serve subscribers close to them with a low transmission power. The remainder of the sub-channels are used with a higher transmission power and can be used by both distant and close subscribers. To minimize interference of high power sub-channels for clients of neighboring base stations the cells are further organized in a way that two adjacent cells do not use the same high power sub-channels. As such, cell edge mobile terminals can be scheduled on the high power tones that are not used or are used with lower transmit power by the neighbouring sectors. This approach is known as fractional frequency re-use (“FFR”) as all base stations use the same frequency band with different power level restriction on different sub-channels. Some tones are used by all the sectors and thus have a reuse factor of one, whereas, other tones may only be used by a third of the sectors and thus have a reuse factor of ⅓.
Known fractional frequency reuse (FFR) schemes include hard and soft FFR schemes. Reuse, refers to the quantity of cells or sectors serviced by a base station. In such existing FFR solutions, however, the reuse factor, i.e., pattern, is fixed. In other words, the reuse arrangement of high power sub-channels and low power sub-channels within a cell can not be changed, even though the conditions within the cell have changed, either for the better or worse. Thus, there will always be some impact on throughput in known systems even when there are no coverage problems. Fixed FFR schemes, in addition, require frequency planning. In soft reuse, a portion of the tones are used with lower power, whereas, in hard reuse, a portion of the tones are not used at all.
A need exists, therefore, for improved FFR methods and systems that allow for the reuse of frequencies based on the detection of coverage problems within the cell.